The purposes of conversion coatings on metal surfaces are to provide corrosion resistance and improve the adhesion of the ultimate coating(s). The conversion coating improves the adhesion of final coating layers such as paints, inks, lacquers and plastics.
Traditionally, chromates have been utilized as conversion coating compounds. Chromates are known to exhibit acceptable performance on different types of metals or alloys, such as aluminum, steel, galvanized steel and zinc-aluminum coated steel. Chromate conversion coatings are typically administered by contacting the metal surface with an aqueous solution containing hexavalent or trivalent chromium ions, phosphate ions or fluoride ions. Serious concerns have been raised, however, regarding the pollution effects of the chromate or phosphate discharged into rivers or waterways by such processes. Due to the high solubility and the strongly oxidizing character of hexavalent chromium ions, conventional chromate conversion coating processes require extensive waste treatment procedures to control their discharge.
While many acceptable chromium-free conversion coatings have been developed, their utility is often limited to only one type of metal substrate. Many industrial operations, however, involve the sequential processing of different types of metals. Unless the undesirable chromium compounds are used, the industrial metal processor is forced to change the conversion coating bath for each different type of metal. This results in unacceptable downtime and higher processing costs.
During operation of an automotive evaporator, for example, moisture condenses on the aluminum fins. If the fin surface is hydrophobic, the condensed water, in the form of droplets, can form bridges between the fins, increasing resistance to air flow and decreasing the efficiency of the evaporator.
Conventional pretreatment of an evaporator to produce a hydrophilic surface involves the formation of a chromate conversion coating followed by a final silicate rinse. It is believed that the chromate coating forms a base for silicate, which imparts hydrophilicity to the surface. The performance of a pretreatment system may be determined by placing an evaporator core on a test stand and "hooking" into an air conditioning system. As the unit operates, water is condensed on the evaporator fins, and an air flow measurement across the fin surface is made. The change in air pressure across the wet fins (Wet Delta P- in inches of water), measured at various times during the test, is an indication of the performance of the hydrophilic coating. A smaller pressure change indicates a better coating.
While initial pressure drop (Wet Delta P) is low for a chromate/silicate process, the pressure drop increases steadily throughout the test. This change in performance over time suggests that the silicate coating is removed during the long term operation of the unit. Since an evaporator must be appropriately sized to adequately perform its function, this decrease in efficiency must be accounted for in the design of the evaporator.
It is desirable, therefore, to have a metal surface treatment that does not utilize a chromate conversion coating. Although non-chromate conversion coatings followed by a silicate final rinse are commercially available, it is also desirable to combine the process of forming a hydrophilic coating into one step, thus reducing water consumption and waste discharge. This is achieved by the composition and process of the present invention.